How a Lead-Acid Battery Works: The Electrochemistry Behind SLI, AGM and EFB
A lead-acid battery is a reversible chemical engine: it converts chemical energy to electricity on discharge and back again on charge. Every property of the battery — its 2.1V cells, its CCA, its aging — traces back to that chemistry.
The discharge reaction
At the negative plate, lead gives up electrons to become lead sulfate. At the positive plate, lead dioxide accepts electrons and also becomes lead sulfate. The electrolyte supplies sulfate ions and is consumed — which is why the acid gets weaker as the battery discharges, a measurable signal of state of charge.
The overall reaction converts lead, lead dioxide and sulfuric acid into lead sulfate and water. The electrons that flow between the two plates through the external circuit are the current that does the work.
The charge reaction
Charging pushes current the other way and reverses everything: lead sulfate on each plate is converted back into lead on the negative and lead dioxide on the positive, and sulfuric acid is regenerated. This reversibility is why a lead-acid battery can cycle thousands of times — but never perfectly, because each cycle leaves a little sulfate behind.
Why 2.1V per cell
The ~2.1V figure is a fixed property of the lead/lead-dioxide/sulfuric-acid chemistry, not a design choice. A "12V" battery is six 2.1V cells in series (≈12.6V when fully charged and at rest). This is also why a battery's resting voltage is a direct read on its state of charge: about 12.6V full, about 12.0V half, and below ~11.9V nearly empty.
The three failure modes
- Sulfation — lead sulfate that is not re-converted hardens into crystals that block the plates, the dominant cause of early death in an under-charged battery.
- Corrosion — the positive grid slowly oxidises, especially with overcharging and heat.
- Water loss — overcharging splits water into hydrogen and oxygen, drying the electrolyte.
This is also why the technology variants matter: AGM immobilises the electrolyte to suppress water loss and improve charge acceptance, and EFB adds carbon to the negative plate for the same reason.
What do people get wrong about how lead-acid batteries work?
- "The acid is the energy." The energy is in the electrodes; the acid is the medium that carries the reaction. Both plates, not the acid, store the energy.
- "12V is exact." It is nominal — a healthy, fully-charged 12V battery rests at ~12.6V, not 12.0V.
- "Discharging to empty is harmless." Deep discharge promotes sulfation, which is why a lead-acid battery left flat dies early.
Key takeaways
- Lead + lead dioxide + sulfuric acid ⇄ lead sulfate + water — the reversible reaction is the whole battery.
- Each cell is ~2.1V; six in series make ~12.6V (a "12V" battery).
- Resting voltage is a direct state-of-charge gauge.
- Sulfation, corrosion and water loss are the three aging mechanisms.
The chemistry in one equation
The whole battery is one reversible reaction — the "double sulfate": Pb + PbO₂ + 2H₂SO₄ ⇌ 2PbSO₄ + 2H₂O. Discharge turns the plates and acid into lead sulfate and water (that is why the acid gets weaker); charge reverses it. The equation explains two practical facts: the electrolyte density tracks state of charge, and sulfation — sulfate that will not convert back — is the chemistry's natural failure direction.
The bottom line
The double-sulfate reaction explains everything downstream — why lead-acid sags in the cold, why it needs charging, and why 2.1V per cell is the chemistry's signature.
Our Interpretation
My read: The single most useful thing to understand about lead-acid is that it dies by slow, reversible chemistry — and most early failures are sulfation from being left under-charged, which is a usage problem, not a manufacturing one.
The reasoning: The reactions are reversible in principle but never perfectly in practice, and the gap between "reversible" and "perfect" is sulfation. A battery that is kept charged lasts years; the same battery left flat for weeks sulfates and dies. This is why maintenance — keeping it charged — matters more than brand.
My view as an editor, not a purchasing guarantee.
An original calculation: why 2.1 V per cell
The cell voltage falls directly out of the reaction’s free energy: the double-sulfate reaction releases on the order of 394 kJ per mole, and E = ΔG ÷ nF = 394,000 J ÷ (2 × 96,485 C) ≈ 2.04 V at standard conditions. The 2.1 V on datasheets is the same number with real concentrations and operating temperature folded in — and six cells in series are exactly why a “12 V” battery sits at 12.6–12.8 V open circuit instead of 12.0.
Frequently asked questions
How does a lead-acid battery work?
It stores energy chemically. During discharge, the lead (Pb) negative plate and lead-dioxide (PbO2) positive plate react with sulfuric acid (H2SO4) to form lead sulfate (PbSO4) and water, releasing electrons as current. Charging reverses the reaction, converting the sulfate back into lead, lead dioxide and acid.
Why is a lead-acid cell 2.1 volts?
It is a property of the lead/lead-dioxide/sulfuric-acid chemistry — the potential difference between the two electrodes is about 2.1V per cell. A 12V battery wires six cells in series (6 × 2.1V = 12.6V).
What buyers should ask
- Is the battery's chemistry and cell count consistent with the nominal voltage claimed (2.1V per cell)?
- Does the intended duty (starting vs deep-cycle vs standby) match the plate design behind the chemistry?
- Are the failure modes (sulfation, shedding, corrosion) understood for the chosen type before purchase?
- Does the vendor substantiate the performance claims with the electrochemistry, not just marketing?
Sources
Cross-verified from battery engineering references and manufacturer technical documentation. The lead-acid electrochemistry is well-established and consistent across sources.
- Battery engineering references — lead-acid electrochemistry and failure modes.
- Manufacturer technical documentation — cell voltage and state-of-charge relationships.
Last reviewed: 2026-09-14